Lithium ion secondary batteries have characteristics of having higher energy density and electromotive force than lead storage batteries and nickel hydride batteries, and thus are widely used as a power source of portable phones, notebook PCs, or the like which require reductions in size and weight. In addition, among the lithium ion secondary batteries, those that use a nonaqueous electrolytic solution in which lithium salts are dissolved in an organic solvent as an electrolyte are the mainstream.
However, the secondary battery that uses the nonaqueous electrolytic solution has a possibility of degradation of charging and discharging cycle life-span characteristics caused by volatilization and diffusion and liquid leakage, and has a risk of internal short circuit caused by precipitation of dendrite that grows in a direction from the negative electrode to the positive electrode. Therefore, in the worst case, the secondary battery may be a cause of accidental fire. In recent years, applying the lithium ion secondary battery to a large-size stationary power source for power storage or a power source for an electric vehicle has been expected, and a further increase in energy density and enhancement in safety are strongly desired.
Here, a system which uses a solid or gel-like electrolyte as an electrolyte has been designed and actively studied. Using such an electrolyte, volatilization and diffusion of an electrolytic solution or liquid leakage is prevented. Therefore, reliability and safety of the battery can be enhanced. Moreover, a reduction in the thickness and lamination of the electrolyte itself become easy, and thus enhancement in processability and simplification of packages are expected.
As the gel-like electrolyte, for example, a fluorine-based polymer such as polyvinylidene fluoride and a polyacrylic polymer such as polymethyl(meth)acrylate are used.
In addition, examples of a lithium salt that is used in combination with the polymers include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2), lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium hexafluoroantimonate (LiSbF6), lithium hexafluoroarsenate (LiAsF6), and lithium tetraphenylborate (LiB(C6H5)4).
In the lithium ion secondary battery, lithium salts demand characteristics such as high chemical stability and thermal stability and low cost. However, it is very difficult to satisfy all the demands. For example, LiPF6 used in the battery of a commercially available product has problems of low thermal stability and easily being hydrolyzed, and LiN(SO2CF3)2 has problems of high cost.
Meanwhile, a method of applying a polymer composite (polyanion type lithium salt) which has a lithium salt as an anionic functional group in a polymer and has functions of both a polymer matrix and the lithium salt to the lithium ion secondary battery has been suggested (refer to PTLs 1 and 2).